1. Field of the Invention
The invention relates to cordierite ceramic bodies for use as catalyst carriers, particularly to cordierite bodies, having ultra-thin web sizes, for use as catalyst carriers for purifying automobile exhaust gas, and particularly to a method for producing these ultra thinwall cordierite structures.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to, reduce the quantities of pollutants from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970""s.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to cordierite ceramics"" high thermal shock resistance. The production of cordierite (2MgO.2Al2O3.5SiO2) ceramics from mineral batches containing sources of magnesium, aluminum and silicon such as clay and talc is well known. Such a process is described in U.S. Pat. No. 2,684,919. U.S. Pat. No. 3,885,977 discloses the manufacture of thermal-shock-resistant cordierite honeycomb ceramics from clay/talc batches by extruding the batches and firing the extrudate to provide ceramics with very low expansion coefficients along at least one axis.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to develop cordierite honeycomb substrates that possess smaller and smaller web or wall sizes. Demand for cordierite monoliths having very thin webs is increasing in response to legislation requiring higher conversion efficiencies in catalytic converters for the automobile market. Thinner webs reduce the mass of the substrate resulting in faster light-off times. In addition, higher geometric surface areas may be achieved without an increase in the mass of the substrate. Another advantage of thin walled substrates is that a lower back pressure may be achieved.
The production of honeycomb substrates with very thin webs is, however, very difficult, when compared with substrates with more conventional geometry. It has been found that when conventional extrusion apparatus are used to produce ceramic honeycombs with web thicknesses of less than 100 xcexcm (i.e., ultra-thinwall bodies), an unacceptably high number of breaks in the web of the cellular extrudate (i.e., areas containing no ceramic material) are observed in the extruded product. It is thought that these breaks in the ceramic material result from one or more particles from the extrusion material plugging a slot in the extrusion die, resulting in a region where batch is restricted from flowing. The number of breaks increases as the slot width decreases, and if the slot width is narrow enough, the number of plugged cells becomes so great that the extrudate does not hold together, but rather the extrusion consists of many small strands of batch material.
Further, the extrusion of ceramic honeycombs articles with very thin webs is made difficult by the reduced ability of the wet extrudate to support its own weight. As a result, slumping of the article is common because of the lower strength of the thin webs.
Another difficulty in the extrusion of ceramic honeycomb articles with very thin webs is that small differences in slot width in the die can lead to problems with differential flow rates of the batch material. This differential flow can result in distortion of the honeycomb cell structure.
A further problem associated with conventional extrusion of ultra thin wall cordierite substrates with high cell density is that a multi-component die design is required since the feedholes for a conventional one-piece die design would be too close together, or too small to be drilled. However, when a composition exhibiting a moderately high degree of radial shrinkage is used, a conventional and much less expensive die design is made possible in order to increase the cell density during firing to a level only possible through the use of a multi-component die approach when using materials giving a low level of shrinkage.
The discovery of a method of making ultra-thinwall cordierite that overcame the aforementioned shortcomings of conventional methods and a method that was capable of being used with conventional extrusion apparatus would be highly desirable and an advancement in the art.
It is therefore a principal object of the present invention to provide an improved method for making ultra-thinwall cordierite ceramics, that produces little, if any discontinuities in the ceramic article and which can be utilized with conventional extrusion apparatus/dies. It has been found that when certain combinations of raw materials are used, the raw materials upon extrusion through a thin slot partially align to form an orientation plane. Specifically, it has been discovered that the combination of platy talc and clay raw materials, when used in the preparation of cordierite honeycomb structures, form, upon extrusion a raw material orientation plane that is generally parallel to the web or wall direction. It follows that the green bodies exhibiting this oriented raw material structure exhibit anisotropic shrinkage; a higher degree of shrinkage in a direction perpendicular to the orientation plane (i.e., thickness of the wall shrinkage) than in any other direction; e.g. the radial and axial directions.
More specifically, this invention relates to a plasticizable raw material mixture for use in preparing a substrate having cordierite as its primary phase with the mixture comprised of a chemical composition, percent by weight, of 11 to 17% MgO, 33 to 41% AlsO3 and, 46 to 53% SiO2. The raw material mixture specifically comprises the following (1) a platy talc having a morphology index of greater than 0.8 and exhibiting a mean particle size of less than about 1 xcexcm when measured by a Sedigraph particle analyzer (Micromeritics); (2) an Al2O3 forming source comprising an alumina having a mean particle size of less than about 5 xcexcm selected from the group consisting of xe2x88x9d-alumina, transition aluminas, aluminum oxide monohydroxide, or aluminum trihydrate having; and, (3) one or more of the components of delaminated kaolin having a mean particle size of less than about 2 xcexcm, calcined kaolin exhibiting a mean particle size of less than about 2 xcexcm and a surface area of greater than about 10 m2/g and/or silica having a mean particle size of less than about 5 xcexcm.
This invention also relates to an extrudable mixture that comprises the above-mentioned raw material mixture and an organic binder system. Still further, this invention relates to a method of producing a green body of cordierite-forming materials comprising preparing a plasticizable raw material mixture as defined above, adding an organic binder system to the mixture and mixing the mixture to form an extrudable mixture, and extruding the mixture to form a substrate having the desired configuration. The green body is dried and fired to yield a ceramic substrate having cordierite as its primary phase.